The present invention relates generally to valve packing and more specifically to an improved valve packing wherein the valve packing is exposed to reversal of pressure differentials.
A valve generally includes a housing with a plurality of members or elements therein. The interior elements are sealed, whether they are stationary or movable, by packings. Generally the packings are exposed to a single polarity of pressure differentials in the normal operation of the valve. For a selected group of valves, the packings may be subjected to a reversal of pressure differentials across the packings. One such valve is an equalizing reservoir cut-out valve 10 illustrated in FIGS. 1-3.
The valve 10 includes a housing 12 having an equalizing reservoir control pipe port 14, an equalizing reservoir port 16, and a control port 18. A ported sleeve 20 is received within bore 22 of housing 12 and includes an annular port 24 communicating the interior of the sleeve 20 to the equalizing reservoir port 16. A pair of annular recesses 26 and 28 in the sleeve 20 are adjacent opposite sides of the annular port 24 and include packings 30 and 32, respectively, illustrated as O-rings. The packings 30 and 32 seal the annular port 24 from the control port 18 and the equalizing reservoir control pipe port 14. Sleeve 20 includes a valve seat 34 which forms a check valve in combination with disc 36 which is biased closed by spring 38. A plunger 40 is slidably received within the sleeve 20 and includes ports 42 interconnecting the check valve and the annular port 24. An annular recess 44 in the plunger 40 includes a packing 46 to seal the control port 18 from the interior ports 42 and 22.
The operation of the equalizing reservoir cut-out valve 10 will now be described. When a high pressure control signal is applied at control port 18 as indicated by the arrow therein in FIG. 1, the plunger 40 is moved to the left opening disc 36 of the check valve. This allows communication between the equalizing reservoir control pipe port 14 and the equalizing reservoir port 16. Generally high pressure fluid from the equalizing reservoir control port 14 will flow through seat 34, port 42 and annular port 24 to the equalizing reservoir port 16 as indicated by the arrows therein. If the pressure in the equalizing reservoir is greater than that of the equalizing reservoir control pipe, the flow between ports 16 and 14 is reversed. In either case, the high pressure signal at control port 18 leaks into annular recesses 28 and 44 and positions the packings 32 and 46 against the left side wall of the annular recesses 28 and 44, respectively. This provides a tight seal preventing the high pressure fluid at port 18 from entering the interior of the valve structure.
Once the high pressure control signal is removed from control port 18, it is generally vented to atmosphere. The flow of fluid from equalizing reservoir control pipe 14 through the valve to the equalizing reservoir port 16 will close the check valve and force the plunger 40 back to the right. The pressure in the equalizing reservoir will enter through equalizing reservoir port 16, annular port 44 and port 42 to create a high pressure between the interior of the plunger 40 and the disc 36 of the check valve. This pressure will leak into the annular recesses 28 and 44 and position the packings 32 and 46, respectively, against the right wall of their annular recesses. This will cause the packings 32 and 46 to seal and prevent fluid from the equalizing reservoir to seep into the control port 18. This condition is illustrated in FIG. 2. If the pressure at equalizing reservoir port 16 is greater than the pressure equalizing reservoir control pipe port 14 plus the force of spring 38, the disc 36 of the check valve will open allowing flow of fluid from the equalizing reservoir port 16 to the equalizing reservoir control pipe port 14. Disc 36 of the check valve will seat itself once the pressure at the equalizing reservoir port 16 equals the pressure of the equalizing reservoir control pipe port 14 plus the force of spring 38.
From the description of FIGS. 1 and 2, it can be seen that the equalizing reservoir cut-out valve 10 includes packings 32 and 46 which are subjected to reversal of pressure differentials thereacross. The packings oscillate between the walls of the recesses in which they are located in response to the pressure differential reversals to provide appropriate seals. After several reversals, the lubrication of the packings is reduced, the compression set of the packings causes decreased sealing contact force, and the recesses in which the packings lie possibly corrode. This results in the packing not moving into the correct sealing position against the side wall of the recesses and equalizing pressure will leak past the packings. This may cause an over-application of brakes if the valve is used as an equalizing reservoir cut-out valve. As is illustrated in FIG. 3, the packings 32 and 46 have moved from a sealed position against the left walls of the respective annular recesses, 28 and 44, respectively, but have not traveled completely to the right wall of the recesses to form a seal. The fluid at equalizing reservoir port 16 leaks to the control port 18. This effectively reduces the efficiency of the equalizing reservoir.
One solution of the prior art to maintain a packing within its annular recess is to provide a port between the base of the recess and one side of the valve structure. This is illustrated in U.S. Pat. No. 2,713,989 to Bryant. Although this method may be used in certain valve configurations, it would be totally inoperable wherein the packing is subjected to reversal of differential pressures. For the differential pressure situation, Bryant illustrates in FIGS. 4 and 5 the introduction of the fluid to aid the movement of the packing within its recess so as to be positioned against the appropriate wall of the recess to form a seal. Although such an arrangement may be appropriate for the valve seat of a plunger, it does not have application to packings in a non-valve seat configuration where they are subject to reversals of differential pressures.